This invention relates to an improved evoked response audiometer for use in the diagnosis of deafness.
The diagnosis of deafness at an early stage in paediatrics is important to enable the early fitting of hearing aids and/or cochlear implants in order to assist language development in a hearing-impaired child. It is also important to be able to diagnose deafness in adults who are unable, due to mental illness or disability, or unwilling, for various reasons, to participate in conventional behavioural deafness testing.
In our U.S. Pat. Nos 4,462,411 (Rickards) and 5,023,783 (Cohen and Rickards), we have described evoked response audiometers which use a continuous auditory tone that is frequency or amplitude modulated, the auditory tone being presented for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the person being tested. An electro-encephalograph (EEG) signal from the scalp of the person is manipulated such that the components due to the modulation carried by the auditory stimulus is extracted and detected.
The modulated auditory stimulus produces separate and distinct evoked potentials in the brain depending on the nature of the modulation. These evoked potentials can be difficult to detect, particularly for low sound levels which are less audible to the person being tested.
It is therefore an object of the present invention to provide an improved evoked response audiometer incorporating an improved modulation technique which produces stronger evoked potentials using low sound level auditory stimulus signals.
The invention provides an evoked response audiometer comprising means for supplying to a patient an auditory stimulus signal consisting of a carrier frequency which is modulated by at least two different forms of modulation such that the stimulus is at least substantially frequency specific, said auditory signal being presented for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the patient, means for sampling the brain potential signals evoked by said auditory signal, and means for analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, said means for supplying said auditory signal being selectively controlled to advance or delay one modulation with respect to the other modulation to cause enhancement of the evoked response to the auditory stimulus.
Research has indicated that the combined modulation of the auditory stimulus enables significant improvements in the detection of the evoked potentials whereby evoked potentials of amplitude large enough for detection will be produced by auditory stimuli of lower sound level, and hence lower subjective loudness.
The invention also provides a method of testing for hearing impairment, comprising the steps of supplying to the patient an auditory stimulus signal consisting of a carrier frequency which is modulated by at least two different forms of modulation so that the stimulus is at least substantially frequency specific, presenting the auditory signal for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the patient, sampling the brain potential signals evoked by said auditory signal, analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, and selectively controlling said auditory signal to advance or delay one modulation with respect to the other modulation to cause enhancement of the evoked response to the auditory stimulus.
In a preferred form of the invention, the auditory stimulus signal is amplitude modulated and frequency modulated, preferably in a periodic manner, such as sinusoidal. The potentials evoked in the brain by amplitude modulation and frequency modulation have been found to differ in phase, indicating different delays in the processing by the auditory system to amplitude modulation and frequency modulation. By compensating for the delay in perception of the amplitude and frequency modulation, the auditory signal compensates for the auditory system process by artificially advancing or retarding in time the amplitude modulation or the frequency modulation relative to each other, resulting in the equalisation of the phase delays occurring in the evoked brain potentials.
Without the necessary equalisation, the response to the amplitude modulated signal and the response to the frequency modulated signal can have a phase relationship which results in response cancellation when the responses are vectorially summed. By compensating for the delays in the actual auditory stimulus, the phase of the two responses can be altered so that the vectorial sum is significantly enhanced beyond the stimulus achieved by the use of amplitude modulation or frequency modulation alone. This enhancement results in a higher detection sensitivity to the stimulus by virtue of an improved signal to noise ratio, and consequently, the hearing threshold determined when using the evoked response audiometer much closer to the true behavioural hearing threshold of the patient under test. As a result, estimations of the true behavioural thresholds from the patients evoked response thresholds are improved.
Depending on the frequency of the carrier, the modulation frequency and the corresponding modulation indexes of the auditory signal, the measured physiological delays will vary. All such delays can be compensated for by adjusting the phase relationship between the AM modulation and the FM modulation of the stimulus signal.
In terms of hearing perception, AM is produced by modulating a pure tone (or sinusoid) whose amplitude is varied in a sinusoidal manner by another sine wave at the modulation frequency. FM is produced by modulating a pure tone whose frequency is varied in a sinusoidal manner by a sine wave at the modulation frequency. When both forms of modulation are combined, the frequency and amplitude can be varied together in a number of subtly different ways. For example, the frequency can be high when the amplitude is high; the frequency can be low when the amplitude is high; the frequency can be midway when the amplitude is high, or the frequency can be midway when the amplitude is low.
The relative phase between the AM signal and the FM signal can be given any value between +/xe2x88x92 about 60xc2x0, depending on the signal parameters, to produce enhanced evoked potentials in the brain of the patient.
The responses to AM and FM stimuli, detected in the overall EEG activity, differ. To improve the detection process both modulation methods are used together and the phase difference between the AM and FM modulations is selected to result in constructive addition of the AM and FM response components. In a preferred form, this occurs when the phase difference between the AM and FM modulations is about 30xc2x0. If the modulation components are about 210xc2x0 apart, cancellation will occur. The AM/FM stimulus in this case would produce no or very little detected response to the stimulus.
The phase relationship between the AM and FM detection processes depends on the mechanics of the ear and brain physiology. It also depends on the modulation indices used. The modulation indices determine how much the carrier amplitude is changed by amplitude modulation and how much the frequency of the carrier is changed by the frequency modulation.
It is expected that different relative phase delays will be required depending on the patient tested, the carrier and modulation frequencies, and the AM and FM modulation indices used. Norms for different age groups and conscious states are determined experimentally. To this extent the solution of the more appropriate phase difference is initially determined empirically. However, once an appropriate phase difference is determined, it can be used for similar patient types and similar signal parameters. The system may be designed to determine and be used for diagnosis of particular hearing problems when the phase delays used for normal patients do not provide a response as expected by those norms. Calculations indicate a difference in the optimum AM/FM phase relationship of about 30xc2x0+/xe2x88x9220xc2x0 would contain any detection loss to less than about 0.1 dB. If the vectors are more substantially out of phase, a loss of up to about 9.5 dB can occur. For different signal parameters, a difference in phase of up to about +/xe2x88x9260xc2x0 may produce similar benefits depending on the relative amplitude of the AM and FM responses. If the relative amplitudes are equal, a gain of up to 6 dB will result (see FIG. 10) but if the relative amplitudes are half each other then a loss of benefit results (see FIG. 11).
Calculations indicate that the combined modulations can result in a typical improvement in the signal to noise ratio of about 3.5 dB compared to the response over that of AM used alone. Since the responses being detected are very small compared to the background noise level, this improvement should be considered to be substantial. This assumes the EEG voltage of an FM response is typically half the EEG voltage of an AM response for the same stimulus level.